TUESDAY, Dec. 11, 2018 (HealthDay News) -- On any given night in America, more than 550,000 people are homeless, and they are being hospitalized in greater numbers, a new study suggests.

Despite expanded Medicaid and increased funds for health care clinics, hospitalizations among this vulnerable population are rising, said lead researcher Dr. Rishi Wadhera. He is with the Smith Center for Outcomes Research at Beth Israel Deaconess Medical Center and Brigham and Women's Hospital, in Boston.

"We think this is related to aging of the homeless population and, potentially, the rise of the opioid epidemic," Wadhera said. "Strikingly, more than half of hospitalizations for homeless adults were for mental health and substance use disorders, which we suspect reflects limited access to and coordination of behavioral health services."

For the study, Wadhera and colleagues looked at the trends of hospitalizations among the homeless in Massachusetts, Florida and California from 2007 to 2013. The study data included more than 185,000 hospitalizations for homeless people and 32 million admissions for the non-homeless. The average age for all participants was 46.

All three states showed a significant increase in hospitalizations among homeless adults: from 294 to 420 per 1,000 homeless residents in Massachusetts; from 161 to 240 per 1,000 in Florida; and from 133 to 164 per 1,000 in California. Most homeless patients were uninsured (42 percent) or insured by Medicaid (32 percent).

Among the homeless, 52 percent were hospitalized for mental illness or substance abuse. This compared with 18 percent for non-homeless people, according to the report published in the January issue of the journal Medical Care.

The homeless patients were less likely to be hospitalized for cardiovascular disease, gastrointestinal illness, injury or poisoning, the researchers said in a journal news release.

Homeless people also spent more time in the hospital -- possibly because discharge decisions were affected by the lack of stable housing. The homeless patients also had lower costs of care, which may be due to differences in the intensity of care. In addition, deaths in the hospital were lower among the homeless individuals, the findings showed.

According to the study's senior author, Dr. Karen Joynt Maddox, of Washington University in St. Louis, "There really is an urgent need to reduce financial and non-financial barriers to the use of ambulatory [outpatient] care, for behavioral health services in particular, to improve long-term management of physical and mental illness for homeless individuals."